1. Field of the Invention
The present invention relates, in general, to X-ray shielding in a photo-detector diode array, and, more particularly, to methods and devices for shielding a front side illuminated photo-detector diode array from cross-talk and spurious back side illumination.
2. Relevant Background
The present application relates to the art of medical diagnostic imaging in which penetrating radiation is received by radiation sensitive detectors. The application subject matter finds particular use in computerized tomographic (CT) scanners and will be described with particular reference thereto. However, the invention may also find use in connection with other diagnostic imaging modalities, industrial quality assurance imaging, airport baggage inspection, X-ray fluoroscopy and the like.
Modern X-ray computer tomography scanners commonly employ several hundred X-ray detectors to convert X-ray energy into visible light and ultimately into electrical signals. A detector is usually composed of a scintillator to convert X-ray energy into light and a photodiode to convert that light into an electrical current. The formats of photodiodes used in CT applications can range from a single element, 1-D array to a multi-element, 2-D array.
Each active photodiode array comprises a series of scintillation crystals arranged on a substrate for converting X-ray radiation into light. Under each scintillator crystal is a back-illuminated photodiode that converts the light emitted from the scintillation crystals into an electrical charge. The electrical charge from the photodiodes is then conveyed via an electrical path to a signal processing circuit. Typically, the converted electrical charge leaves each photodiode via electrical connections through a plurality of paths in a substrate to the processing circuitry. The substrate serves both as a supporting mechanical foundation for the circuitry and the photodiode assembly, and as a shield to protect the processing circuitry from stray radiation.
FIG. 1 provides a plan and side view of a highly abstract rendition of a typical photodiode array 100 as is known in the art. A scintillator crystal 110 is typically a six sided cube wherein the one transparent face is bonded to a photodiode 140. The juncture between the scintillator crystal 110 and the photodiode 140 is normally a p+ on n− mating 135. The remaining sides of the crystal 110 are covered with an optically reflective material that facilitates channeling the light generated by the crystal to the transparent face and ultimately to the photodiode 140 below. The photodiode is thereafter connected to processing circuitry 160 via a bonding layer 150 or electrical paths amidst a substrate 158. Between each scintillator crystal arranged on the array 100 is a gap or septa 120. Interposed in the gap and typically extending some distance both above the plane of the scintillator crystals 110 and into the septa 120 are elements of an inter-scatter grid 170.
The inter-scatter grid 170, which is opaque to X-rays, serves to reduce X-ray cross-talk between adjacent scintillator crystals 110. Cross-talk occurs when an X-ray directed at a particular scintillator pixel possesses a trajectory so as to falsely impact an adjacent crystal. The result is noise, false imaging and/or ghosting. By placing an inter-scatter grid 170 in the septa 120 between the crystals, the likelihood of X-ray cross-talk is reduced. FIG. 2 shows an expanded depiction of the septa region 120 between two scintillator crystals 110 of FIG. 1. The inter-scatter grid 170 extends into the septa only a sufficient distance so as to prevent cross-talk between scintillator crystals 110. Inter-scatter grids 170 do not completely occupy the septa 120 between the crystals 110 and the underlying photodiodes 140. The remaining space provides a means by which stray X-rays can impact adjacent photodiodes or travel through the scintillator 110/photodiode 140 region and impact the processing circuitry 160 itself
In the present state of the art of front side or back side photo detector diode arrays, electrical cross-talk can be minimized by using trench etching filled with oxide and poly-silicon to electrically isolate between diodes in the array. However, this material does not provide any isolation for X-rays traveling from one diode area to another, nor is this structure effective at stopping visible photons from penetrating to adjacent photodiodes. Also there has to be added shielding placed behind the photo detector array in order to protect the processing circuitry mounted behind the photodiode array that processes the signals from this detector.
The processing circuitry associated with each electrical signal from each active photodiode element is typically a Complementary Metal-Oxide Semiconductor (“CMOS”) chip. CMOS is a major class of integrated circuits. CMOS chips include microprocessor, microcontroller, static RAM, and other digital logic circuits. A wire bond typically connects a top surface bond pad on one end of the photodiode to an external connection on the CMOS chip. The conductive path to the electronics is completed using various design options.
The wire bond density becomes acute for 2-D arrays. A conductive trace from each inner photodiode element in a 2-D array must be connected to the “outside world”. This trace is usually included on the photodiode surface between rows of active photodiode elements. One trace is required per element and each trace usually terminates in a bond pad at an end of the 2-D array. Wire bonds from each trace are then made to external connections.
One problem of computer tomography relates to degradation of the signals as they travel over the long bus system between the radiation detectors and the signal processing circuitry.
CT scanners operate in a sea of extraneous radio frequency electromagnetic signals, the frequencies of which vary over a wide band. Sources of extraneous signals include X-rays passing through the septa, nearby operating electrical components, equipment, signals from other detectors, and the like. The extraneous analog signals are superimposed on and mix with the analog signals from the detectors. The superimposed extraneous signals appear as noise and fictitious data when reconstructed into images. The resulting images are degraded by noise, ghosting, and other artifacts.
Frequently stray X-rays find themselves traveling down the septa 120 rather than impacting a scintillator crystal 110. Some of these stray X-rays are blocked from the CMOS circuitry by a radiation shield imbedded in the supporting substrate. The substrate is typically a ceramic layer that provides both structural integrity for the array as well as a means for shielding the CMOS circuitry from stray radiation. In the absence of such shielding, the X-ray may penetrate completely through the photo detector die completely and continue on to any detector electronics mounted behind the detector chip producing spurious signals in those circuits. A shielding layer of a high atomic number such as tungsten is therefore typically imbedded in the substrate. Such a shield is expensive to implement. The number of interconnections between the photodiodes and the CMOS circuitry has also been long appreciated as a design limitation. One solution to this limitation is to bond the CMOS circuitry directly to the photodiodes via solder bumps. The result of this alternative means of attaching the CMOS circuitry has the significant advantage of eliminating the need for the expensive ceramic substrate. However, with this advantage comes an equally detrimental effect of exposing the CMOS circuitry to stray radiation.
The present invention contemplates an improved method and apparatus which overcomes the above, and other, referenced problems.